Continuously cultured MCF-7 human breast cancer cells have been recently reported to be responsive to both androgens and estrogens. Studies in this laboratory have confirmed the presence of several enzymes in this cell line that allow the intracellular synthesis of biologically active androgens and estrogens from extracellular precursor steroids. The objectives of this proposal are to 1) identify the spectrum of steroids capable of being transformed by this cell line to active sex steroids and 2) determine whether or not intracellular sex steroid formation can maintain hormone-dependent events in this cell line. Studies include the determination of 5 alpha-reductase, 3-hydroxysteroid dehydrogenase, aromatase, and 17 beta-hydroxysteroid dehydrogenase activities in both whole cultures and cell free MCF-7 systems under a variety of substrate conditions. They will also involve studying the effects of various single and combined steroid treatments upon cell protein, steroid enzyme, and DNA contents, as well as alterations in cellular progesterone, estrogen, and androgen receptor levels. Several studies report that some human breast tumors are capable of forming androgens and/or estrogens from extracellular precursor. However, investigations into the relative importance of these phenomena in regulating sex hormone dependent cell growth have been hampered by the lack of a suitable model. Our studies now indicate that MCF-7 cells should provide such a system. We anticipate that the results of this study will provide additional insights into the hormonal regulation of human breast cancers, which may eventually lead to improved cancer therapies.